For obvious reasons, this week is devoted to words related to tsunamis. I have had to put some of this science together myself, trusting readers to provide any needed correction.

Before we can properly define 'tsunami', we must define a scientific concept.

shallow-water wave – a wave whose depth is a very small, in comparison to its wavelength

The concept is relevant because with a shallow-water wave, one can simplify the formula for determining how fast the wave moves. Basically, only one factor is relevant: wave speed is proportional to the square root of wave depth. The deeper the wave, the faster it moves.

Obviously, a shallow-water wave can be one of small depth, or one of great wavelength. This point will lead us to the definition of 'tsunami', tomorrow.

The dictionaries will tell you that a tsunami is a kind of 'tidal wave' or storm wave (Webster's, Hutchinson; Wordnet), or a 'large', 'great' or 'high' wave. (Webster's, OED; AHD). That is shameful. In fact a tsunami it has nothing to do with the tides or storms. And its strike is insidious precisely because, in the open ocean it is not 'large' at all, it can barely be noticed.

We said that a "shallow-water wave" is one whose depth is a very small, in comparison to its wavelength. That of course is achieved if a wave has very long wavelength. Such a wave is called a tsunami.

tsunami – a series of waves of extremely long wave length and long period (generated in a body of water by an impulsive disturbance that vertically displaces the water)¹

For two reasons, a tsunami will strike with little warning. First, it crosses deep ocean very quickly, roughly that of a jet plane, and with little energy loss. (Reason: with such great wavelength, even in deep ocean it still acts as a shallow-water wave.²) Second, a bit of reflection shows that a tsunami -- however big it may be when it hits the land -- is almost undetectably small while it is speeds across the ocean. As you've read, it takes special sensors to detect it there. Ships at sea will not even notice it, and hence provide no radio warnings, and we do not read of tsunami damage to ships at sea.

Indeed, invisibility-at-sea is the source of the name tsunami, meaning "harbor wave" in Japanese. Fisherman who had had a calm and peaceful voyage, with no extraordinary waves, would be baffled to return and find the port devastated. The disastrous wave, it would seem, was one that had struck only in the harbor.

A tsunami becomes noticeable only when it slows, in shallow coastal waters. There its size depends on harbor configuration, and may not be particularly impressive. But the tsunami continues inexorably, and simply does not stop.

¹Source: US National Oceanic & Atmospheric Administration, which appears they have taken the accepted scientific definition. A tsunami can have a wavelength in excess of 100 km – some sources say much more – and period of several minutes to an hour. Contrast typical wind-driven swells along a beach: wave length of 150 m and period of about 10 seconds.

²For such a wave, speed = √(g*depth), where g = acceleration of gravity, 9.8 m/sec/sec. Work it out and you'll find a tsunami travels through 5,000-meter ocean (which is not atypical) at almost 800 km/hr, or 500 mph.

This message has been edited. Last edited by: wordcrafter, January 04, 2005 08:41

We've seen that a tsunami is generated in a body of water by an impulse that vertically displaces the water. What could that impulse be? Destructive tsunamis are most commonly caused by undersea earthquakes (less commonly by submarine landslides, infrequently by submarine volcanic eruptions and very rarely by a large meteorite striking the ocean from above).

It follows that destructive tsunamis will most often occur in the most earthquake-prone area of the ocean. That area is the Pacific Ocean, in which about 90% of the world's earthquakes occur.¹ That Pacific area has been given a special name.

ring of fire – an extensive zone of volcanic and seismic activity that coincides roughly with the borders of the Pacific Ocean

¹The next most seismic region, 5-6% of earthquakes, is the Alpide belt, extending from Mediterranean region eastward through Turkey, Iran, and northern India. I assume that since it is not oceanic, earthquakes there would not generate tsunamis.

bathemetric – regarding measurement of the depths of an ocean or other body of water

[T]he Indian Ocean has none of the deep-sea tsunami detection equipment. Still, Titov said, a rapid forecast and alert system based just on the seismic and bathymetric (sea floor topography) data could have saved lives. There are six tsunameters deployed in the Pacific Ocean today, Bernard said, offering the "bare minimum" of an early warning system. "We probably need more like 20," he said. – Tom Paulson, The Seattle Post-Intelligencer, Dec. 28, 2004, quoting cutting-edge experts at the NOAA's Pacific Marine Environmental Laboratory in Seattle